Audiofrequency amplifier



April 17, 1928.

1 1,666,412 H. P. DONLE AUDIO FREQUENCY AMPLIFIER Filed Oct. 16. 1926 VENTOR Patented Apr. 17, 1928.

* i UNITED STATES PATENT OFFICE.

' mono I."DONLE, OF MERIDEN, CONNECTICUT, ASSIGNOB TO iDONLE ELECTRICAL -.PRODUCTS CORPORATION, OF MERIDEN, CONNECTICUT, A CORPORATION OF CON- NECTICUT.

AUDIOFREQUENCY AMPLIFIER.

Application filed October 16, 1926. Serial No. 141,898.

My invention relates to means for improving audio frequency, amplifiers such as are used in receivers, public address systems, and as speech input amplifiers in broadcast transmitters. i

The main object of my invention is to stabilize inulti stage amplifiers so as to prevent self-oscillation.

A more specific object is to prevent self-os- II cillation when the batteries used for anode voltage supply become partially run down thus increasing their internal resistance which is common to all anode circuits and produces an undesirable coupling between ll these circuits tending to produce self-oscillation.

Referring to the drawings:

In Fig. 1, I show a three stage audio frequency amplifier employing coupling de- 8 vices of the type described in my copending application #98451 and shown in detail in Fig. 2. It will be understood, how

ever, that my invention is also applicable to the usual impedance or transformer coull pled amplifiers.

In Fig. 1, is the detector tube whose grid 6' is usually connected through the condenser 7 and the leak 8 of the last tuned circuit of the receiver (not shown). The

'0 plate 9 of this tube 5 is connected through one coil of the coupler 11 to some point, such as 45 volts, of the common anode battery (not shown). The plate 9 is als con-' nected through the coupling 0011(1811181 12 u to the grid 13 of the first amplifier tube 14. This grid 13 is also connected through the coil 15 of the coupling device 11, and the resistance 16 to the negative terminal of the filament 17.

o The connections of the two following coupling devices 18 and 19 and their associated tubes 20 and 21 may be, similar except for the omission of a resistance similar to 16.

The plate circuit of the tube 21 contains.

the out-put impedance 22 and the blocking condenser 23 whose purpose is to prevent the flow of the comparatively heavy direct current component of the anode current through the windings of the reproducing device 24.

.9 It will be observed that provision is preferabl made for the use of a grid biasing or battery with the tube 21.

The resistance 16 may have a value of say between 10,000 and 30,000 ohms. 20,000 s a good value.

it has been found that when 'self-oscil is usually of the order of several hundred thousand ohms so that the insertion of a resistance of say 20,000 ohms only effects a small percentage increase ofinipedance.

Upon the other hand the resistance componeut of this impedance is usually of the order of 2000 ohms so that the insertion of a resistance of 20,000 ohms may increase the damping factor by a ratio of 10:1 thus greatly reducing any tendency to oscillate.

When each stage of the amplifier has an extremely high amplificaton factor, it may be desirable 'to insert resistances of the order mentioned in the inductive grid circuits of more than one stage.

I claim:

1. A multi-stage audio frequency amplifier system including three element vacuum tubes having an inductive and capacitative coupling between adjacent tubes and a resistance inserted in series with the input circuit of the second tube, the inserted resistance being much greater than the resistance of the grid inductance and the impedance of said inserted resistance being much smaller than the impedance of the grid inductance at average audio frequencies.

2. An audio frequency amplifier coupling between two three element vacuum tubes comprising a capacity for transferring part of the energy from the plate of the first tube to the grid of the second tube, an iron core transformer having a closed magnetic circuit and coils connected to the plate and grid circuits and providing substantial mutual inductance for transferring the balance of the energy from the first to the second tube and a resistance inserted in circuit with the grid average an io frequencies.

3. An audio frequency amplifier system 1 having vacuum tubes with a transformer and condenser coupling between the late of one tube and the grid of the next m e, a resistance in the grid circuit of. the second tube,

said resistance having an impedance only a small fraction of the impedance of the grid circuit, said resistance being several times that of the total resistance of the remainder of said circuit.

'4. In an audio frequency amplifier having an inductive grid circuit, a resistance inserted in said circuit, said resistance having an impedance which is only a small fracton ot' the impedance of said circuit, but a resistance several times that of the total resistance of said circuit.

5. A multi-sta'ge audio frequency amplifiersystem including relay elements, inductive and capacitative coupling between said elements and a resistance inserted in series with the in-put circuit of the first element, the impedance of the inut circuit of the first stage being of the or er of several hundred thousand ohms and the inserted resistance being of the order of 20.000 ohms HAROLD P. DONLE. 

